With the recent rapid development of portable and cordless electronic devices such as audio-visual (AV) devices and personal computers, there is an increasing demand for secondary batteries or batteries having a small size, a light weight and a high energy density as a power source for driving these electronic devices. Under these circumstances, lithium ion secondary batteries having advantages such as a high charge/discharge voltage and a large charge/discharge capacity have been noticed.
Hitherto, as positive electrode active substances useful for high energy-type lithium ion secondary batteries exhibiting a 4 V-grade voltage, there are generally known LiMn2O4 which has a spinel structure, LiMnO2, LiCoO2, LiCo1−xNixO2 and LiNiO2 which have a rock-salt type structure, or the like. Among these positive electrode active substances, LiCoO2 is more excellent because of a high voltage and a high capacity thereof. However, LiCoO2 has the problems such as a high production cost owing to a less supply amount of raw cobalt materials and a poor environmental safety upon disposal of batteries obtained by using the substance. In consequence, there have now been made earnest studies on lithium manganate particles with a spinel type structure (basic composition: LiMn2O2; this is similarly applied to the subsequent descriptions) which are produced by using, as a raw material, manganese having a large supply amount, a low cost and a good environmental compatibility.
As is known in the art, the lithium manganate particles may be obtained by mixing a manganese compound and a lithium compound at a predetermined mixing ratio and then calcining the resulting mixture in the temperature range of 700 to 800° C.
When using the lithium manganate particles as a positive electrode active substance for lithium ion secondary batteries, the resulting battery has a high voltage and a high energy density, but tends to suffer from the problems such as poor charge/discharge cycle characteristics. The reason therefor is considered to be that when the battery is subjected to repeated charge/discharge cycles, the crystal lattice of the lithium manganate particles used therein is expanded and contracted owing to desorption and insertion behavior of lithium ions in the crystal structure to cause change in volume of the crystal, resulting in occurrence of breakage of the crystal lattice or dissolution of Mn in an electrolyte solution.
At present, in the lithium ion secondary batteries using the lithium manganate particles, it has been strongly required to suppress deterioration in charge/discharge capacity due to repeated charge/discharge cycles, and improve the charge/discharge cycle characteristics, in particular, under high-temperature and low-temperature conditions.
In order to improve the charge/discharge cycle characteristics of the batteries, it is required that the positive electrode active substance used therein which comprise the lithium manganate particles has an excellent packing property and an appropriate size, and further is free from elution of Mn therefrom. To meet the requirements, there have been proposed the method of suitably controlling a particle size and a particle size distribution of the lithium manganate particles; the method of obtaining the lithium manganate particles having a high crystallinity by controlling a calcining temperature thereof; the method of adding different kinds of elements to the lithium manganate particles to strengthen or reduce a bonding force between the crystals; the method of subjecting the lithium manganate particles to surface treatment or adding additives thereto to suppress elution of Mn therefrom; or the like.
Conventionally, it is known that aluminum as one of the different kinds of elements is incorporated in the lithium manganate particles (Patent Documents 1 to 6). In addition, it is known that an anti-sintering agent having a melting point of not higher than 800° C., in particular, phosphorus, a phosphorus oxide or a phosphorus compound, is added to the lithium manganate particles upon calcination of the particles to impart thereto the effect of preventing elution of Mn therefrom. In Patent Documents 7 and 8, there have been respectively described lithium manganate particles having a coating layer for preventing elution of Mn therefrom which is obtained by adding phosphorus to the particles and calcining the resulting mixture in an oxidation atmosphere at a temperature of 650 to 900° C. (Patent Document 7), and lithium manganate to which a boron compound and a phosphorus compound are added to suppress the reaction with an electrolyte solution (Patent Document 8).
More specifically, there are respectively described the method of incorporating a Ca compound and/or an Ni compound as well as an Al compound into lithium manganate particles (Patent Document 1); the method of incorporating Al into lithium manganate particles in which positions of peaks of respective diffraction planes as observed in X-ray diffraction analysis thereof are defined (Patent Document 2); the method of incorporating a different kind of element such as Al into lithium manganate particles and conducting calcination of the lithium manganate particles at multiple separate stages (Patent Document 3); lithium manganate obtained by incorporating Al into lithium manganate particles, which has a specific surface area of 0.5 to 0.8 m2/g and a sodium content of not more than 1000 ppm (Patent Document 4); lithium manganate obtained by incorporating a different kind of element such as Al into lithium manganate particles which comprises crystal particles having a half value width of (400) plane of not more than 0.22° and an average particle diameter of not more than 2 μm (Patent Document 5); lithium manganate obtained by incorporating a different kind of element such as Al into lithium manganate particles which comprises crystal particles having a crystallite size of not less than 600 Å and a lattice distortion of not more than 0.1% (Patent Document 6); lithium manganate obtained by adding phosphorus to raw particles and calcining the resulting mixture in an oxidation atmosphere at a temperature of 650 to 900° C. (Patent Document 7); and lithium manganate to which a boron compound and a phosphorus compound are added to suppress the reaction with an electrolyte solution (Patent Document 8).    Patent Document 1: Japanese Patent Application Laid-Open (KOKAI) No. 2000-294237    Patent Document 2: Japanese Patent Application Laid-Open (KOKAI) No. 2001-146425    Patent Document 3: Japanese Patent Application Laid-Open (KOKAI) No. 2001-328814    Patent Document 4: Japanese Patent Application Laid-Open (KOKAI) No. 2002-33099    Patent Document 5: Japanese Patent Application Laid-Open (KOKAI) No. 2002-316823    Patent Document 6: Japanese Patent Application Laid-Open (KOKAI) No. 2006-252940    Patent Document 7: Japanese Patent Application Laid-Open (KOKAI) No. 9-259863    Patent Document 8: Japanese Patent Application Laid-Open (KOKAI) No. 2001-52698